A first method to carry out this recovery consists in the absorption of the maleic anhydride with water: it reacts with the water to form maleic acid which is thus in form of a solution capable of attaining a concentration up to about 50%. After separation of the water by distillation maleic anhydride is recovered by dehydration of the maleic acid.
The effectiveness of this process has been improved by adding a partial condenser upstream of the water washing, since by lowering the temperature of the gaseous mixture below the dew point of the maleic anhydride, part thereof condenses and the recovery is only devoted to the residual portion remaining in the gaseous phase.
However this type of recovery is affected by a number of drawbacks, among which the following are worthy of note:
the distillation of the water and the dehydration of the maleic acid cause a great energy consumption; PA0 part of the product is lost owing to the conversion of maleic acid into fumaric acid; PA0 in turn the fumaric acid is solid whereby the absorption apparatus is fouled thereby, thus causing the plant to be often stopped for the washing operations. PA0 lower amount of byproducts and mainly of fumaric acid, which causes a higher purification yield in comparison with the standard process of absorption with water; PA0 lower energy consumption, since the evaporation of relevant amounts of water is avoided; PA0 lower amounts of aqueous effluents, mainly originated from the water washing for the solid removal, such as the fumaric acid, a depuration treatment being necessary for these effluents.
Alternatively it has been proposed a long time ago to effect this recovery through absorption in an organic solvent.
Such an organic solvent, preferably dibutyl phthalate, was firstly used for the absorption of mixtures of phthalic anhydride and maleic anhydride resulting from the oxidation of hydrocarbons, such as for example o-xylene, benzene and naphthalene (see for instance U.S. Pat. No. 2,942,005 and other patents).
Thereafter the attention was focused on the recovery of maleic anhydride only, obtained through the catalytic oxidation of butane, butene or mixtures of C.sub.4 hydrocarbons. U.S. Pat. No. 3,981,680 to Chevron Research Co. discloses a process according to which the reaction gaseous mixture is contacted with a liquid phase at least mainly consisting of a dialkyl phthalate having 4 to 8 carbon atoms in each alkyl group and a total of 10 to 14 carbon atoms in the two alkyl groups taken together.
In turn U.S. Pat. No. 4,118,403 to Monsanto Co. discloses a like process, except that the liquid absorbing phase is added with a critical amount of phthalic anhydride permitting the temperature to which the solvent may be heated to be controlled and leading at the very end to a recovery of maleic anhydride from the gaseous reaction mixture higher than 98%.
Among the advantages originating from the organic solvent process the following are worth mentioning.
Thus, as a matter of fact, in the solvent process as practiced to date the mixture of reaction gases is passed through an absorption column in which it comes into contact with the organic solvent in liquid form by which the maleic anhydride is absorbed.
The thus enriched solvent is passed to a separating apparatus for the maleic anhydride, the latter being transferred to the purification step, whereas the impoverished solvent is recycled to the absorption column. However, before such a recycling, the solvent in which heavy products build up is washed with water.
Even by adopting the above mentioned measures some problems and drawbacks still exist among which, firstly, that of the even modest absorption of water by the solvent before it is passed to the separation of the maleic anhydride and recycled to the absorber. This water, as a matter of fact causes the solvent to be decomposed and a certain amount of fumaric acid to be formed at expenses of the maleic anhydride to be recovered, whereby not only the recovery yield is reduced, but the problems of the fouling of the absorber, of the maleic anhydride separator and of the heat exchanger due to the solid fumaric acid being deposited still are present although in a lower degree.